1. Introduction

A Ti-Zr-V type non-evaporable getter (NEG) coating, which was developed at CERN, is a breakthrough vacuum technology because it makes the vacuum chamber wall a getter surface by a low activation temperature around 180-300 ℃ [1, 2]. The oxide surface layer of the NEG coating reduces to metallic Ti, V, and Zr by the activation and thus becomes a getter. In more detail, the displacement reaction of Zr on oxidized Ti or oxidized V is the reason for the low activation temperature of the Ti-Zr-V coating [3]. On the other hand, the deterioration of the getter coating, which means the decrease of the sticking factor, is generally said to be caused by the increase of the oxygen concentration in the film. The Zr oxide of thin NEG films such as 30 nm or 87 nm was difficult to reduce to the metallic Zr compared to that in thick films such as 203 nm or 1100 nm after the same activation condition [4]. Those activation and deterioration mechanism analyses were performed by photo electron spectroscopy with an X-ray tube or synchrotron radiation as a photon source.

2. Measurements

We performed a sequence of measurements with XPS to understand more detail about the activation and deterioration mechanism. The sample of a titanium plate with Ti-Zr-V coating of 1 μm thickness was prepared. The NEG was coated on the titanium plate sample by the DC magnetron sputtering with NEG alloy [5]. The sample was set in the surface science station in the BL23SU of SPring-8. At first, the XPS measurements for the sample surface were performed during the sample temperature was raised to 250℃. After that, the XPS was subsequently performed during the injection of oxygen gas into the chamber while keeping the sample temperature at 250℃, which corresponds to the accelerated deterioration test. This would be the first time that the surface oxidation process was measured in situ by XPS. After that, the depth profile of the sample was measured with another XPS apparatus with an X-ray tube by argon etching.

3. Results and discussions

During the temperature rise measurement, the Zr oxide was shifted to the more oxide spectra (larger binding energy) at the same time that the Ti and V spectra shifted from the oxide to the metal. After that, when the Ti and V become more metallic with the higher temperatures, Zr also moves to the metallic side. This suggests that the surface Zr gets the oxygen from Ti oxide and V oxide at the first stage of the activation and the oxygen of the Zr oxide would diffuse to the bulk in the continuous temperature rise. During the oxygen gas injection at 250℃, the oxidation proceeded in the order of Zr, Ti, and V. These results agree with the order of the stabilities of metal oxides from the Gibbs free energy [3]. The depth profile of the sample saturated by oxygen shows that oxygen mainly forms in Zr oxide in the NEG coating. The second-biggest oxide component in the coating was Ti oxide. The V shows the metallic spectra in the coating interior even after the NEG coating was saturated by oxygen.

4. Conclusion

XPS measurement of the activation and oxidation process was performed in situ at BL23SU of SPring-8. During the oxygen gas injection at 250℃, the surface oxidation proceeded in the order of Zr, Ti, and V. The depth profile of the NEG coating sample, which was saturated with oxygen, revealed that the concentrated oxygen in the coating exists in the form of mainly Zr oxide and Ti oxide in the second place.

References

[1] C. Benvenuti, et al., VACUUM 50, 57 (2001).

[2] O. B. Malyshev, et al., Journal of Vacuum Science & Technology A 27, 321 (2009).

[3] E. Belli, et al., Physical Review Accelerators and Beams 21, 111002 (2018).

[4] C-C. Li, et al., Thin Solid Films 515, 1121 (2006).

[5] J. Kamiya, et al., e-Journal of Surface Science and Nanotechnology 20, 107 (2022).